1. Field of the Invention
This invention relates to a process for the preparation of .alpha.,.beta.-unsaturated aldehydes and substantially no .alpha.,.beta.-unsaturated acids. More specifically, this invention relates to a process for the preparation of .alpha.,.beta.-unsaturated aldehydes and substantially no .alpha.,.beta.-unsaturated acids by a vapor phase catalytic oxidation controlled to yield from 20 to 80% conversion of said aldehydes.
2. Prior Art
Numerous catalysts for the vapor phase oxidation of olefins such as propylene, butenes, isobutenes, amylenes, etc., into .alpha.,.beta.-unsaturated aldehydes, acids and nitriles have been developed in recent years. These catalysts have broadly included the molybdates, tungstates and vanadates of iron, cobalt, nickel and bismuth. Typical of this art are U.S. Pat. Nos. 3,911,089; 3,855,308; 3,642,930; 3,799,978; 3,825,600; 3,907,712; 3,624,146; 3,959,384 and Canadian Pat. Nos. 930,359 and 982,142.
Characteristic of these patents are the unusual number of elements disclosed to have catalytic activity within the broad spectrum of the compositions. For example, U.S. Pat. No. 3,911,089 discloses no less than 27 elements other than oxygen which may be a portion of, or are required in, the catalyst composition. These include one or more representatives from all of the eight groups of the periodic table and all of the periods of the table from 2 through 6, inclusive. Similarly, U.S. Pat. No. 3,855,308 discloses 19 elements plus 3 more which can be present in the form of substrates for the catalyst to be deposited upon. U.S. Pat. No. 3,642,930 mentions 17 different elements. U.S. Pat. No. 3,799,978 discloses 12 different elements plus a number of others in the form of substrates, as do U.S. Pat. Nos. 3,825,600 and 3,907,712. Not only is the art extraordinarily broad in terms of the possible number of elements which can compose the catalyst and its support, but it is, if anything, even broader in terms of the range of proportions of these elements. For example, U.S. Pat. No. 3,642,930 discloses that the catalyst could be greater than 97% pure bismuth oxide modified with less than 1 atom percent each of iron, molybdenum and an alkali metal or greater than 97% pure iron oxide modified with less than an atom percent bismuth, molybdenum and an alkali metal or greater than 97% pure molybdenum oxide with less than an atom percent of bismuth, iron and an alkali metal or 97% pure cobalt oxide or 97% pure nickel oxide. The catalyst could also encompass the nearly pure phosphates of iron, cobalt, nickel, bismuth or molybdenum as well as the whole range of possible mixed oxides such as iron, cobalt, nickel and bismuth molybdates both as pure compounds and as mixtures in all proportions with one another as well as essentially pure potassium nickel oxides, calcium cobalt oxides, etc. U.S. Pat. No. 3,642,930 is more or less characteristic of the above cited patents in the broad range of compositions to which it is directed.
U.S. Pat. No. 3,624,146 discloses that even minor modifications in the composition of a catalyst can dramatically change its effectiveness. The method of preparation of the catalyst, its temperature of heating, the particular crystalline modification and the structural arrangement of the catalyst have a dramatic impact on its selectivity and activity. Table I of said patent discloses that freshly prepared cobalt molybdate has very poor selectivity toward the desired .alpha.,.beta.-unsaturated aldehydes and acids, namely acrolein and acrylic acid, and primarily catalyzes the oxidation of propylene to carbon monoxide, carbon dioxide and acetic acid. Simply by heat-treating this same catalyst at a temperature of 1200.degree. F. the selectivity for acrylic acid is improved by a threefold factor whereas that for acrolein is improved by a dramatic twentyfold factor. By substituting as little as 10 atom percent of the cobalt in the structure with bismuth, an additional threefold improvement is achieved in the case of acrolein and a 50% improvement in the acrylic acid selectivity. If the bismuth concentration is now further increased to a range of 20% of the cobalt concentration, the increase in selectivity toward acrylic acid is dramatically reversed whereas that toward acrolein continues to improve. Specifically, relative to the 10 atom percent bismuth composition, the selectivity toward acrylic acid has been dropped by a fourfold factor, whereas that toward acrolein has been improved by a further factor of two-and-a-halffold.
Thus, it is clear that the extremely wide range in chemical compositions characteristic of this body of art teaches very little in respect to when the literally tens of millions of possible structures and compositions within the scope of each patent should be employed to accomplish any particular reaction with a high selectivity and yield.
U.S. Pat. No. 3,959,384 discloses the propylene oxidation by a process that increases the single pass yield of acrolein. This process, however, like that of the aforesaid references, results in the production of 3 to 10% acrylic acid.
In the preparation of acrolein by the catalytic oxidation of propylene the accompanying formation of acrylic acid with the product poses a serious disposal problem. The acrolein and other desired products may be separated from the product stream which may contain acrolein, acrylic acid, acetic acid, acetaldehyde, other condensable impurities and water by adsorption. Distillation may be used for further separation of e.g., acrolein, but the acid present in the final dilute aqueous acid stream cannot be recovered economically. The aqueous acid stream may be incinerated; however, the very high water content and low fuel content require large amounts of costly fuel. The aqueous acid stream may be treated in an activated sludge type biochemical process. However, the investment in biochemical waste disposal equipment would be extremely high in even a medium size acrolein plant. The loss itself of even a 5% yield to acrylic acid is a matter of no minor consequence in view of the rapidly escalating cost of hydrocarbons.
The art, taken either individually or together fails to show a process for oxidizing propylene wherein there is achieved high yields of .alpha.,.beta.-unsaturated aldehydes coupled with very low concentrations of subsequent oxidation products of these aldehydes such as the .alpha.,.beta.-unsaturated acids.